linux-kernel - Re: [perfmon2] comments on Performance Counters for Linux (PCL)

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Date:	Fri, 29 May 2009 09:24:30 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, eranian@...il.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Philip Mucci <mucci@...s.utk.edu>,
	LKML <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Maynard Johnson <mpjohn@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	perfmon2-devel <perfmon2-devel@...ts.sourceforge.net>
Subject: Re: [perfmon2] comments on Performance Counters for Linux (PCL)

Ingo Molnar writes:

> * Corey Ashford <cjashfor@...ux.vnet.ibm.com> wrote:
> >> So you're suggesting to artificually strech periods by say 
> >> composing a single overflow from smaller ones, ignoring the 
> >> intermediate overflow events?
> >>
> >> That sounds doable, again, patch welcome.
> >
> > I definitely agree with Stephane's point on this one.  I had 
> > assumed that long irq_periods (longer than the width of the 
> > counter) would be synthesized as you suggest.  If this is not the 
> > case, PCL should be changed so that it does, -or- at a minimum, 
> > the user should get an error back stating that the period is too 
> > long for the hardware counter.
> 
> this looks somewhat academic - at least on x86, even the fastest 
> events (say cycles) with a 32 bit overflow means one event per 
> second on 4GB. That's not a significant event count in practice. 
> What's the minimum width we are talking about on Power?

32 bits, but since the top bit is effectively a level-sensitive
interrupt request, the maximum period in hardware is 2^31 counts.

However, I already support 64-bit interrupt periods (well, 63-bit
actually) on powerpc by only calling perf_counter_overflow() when
counter->hw.period_left becomes <= 0, and arranging to set the
hardware counter to 0 if counter->hw.period_left is >= 0x80000000.
It's a tiny amount of code to handle it, really.

Paul.
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